Electron Distribution in BaTi0.98Zr0.02O3 Piezoceramic Using X-ray Structure Factors

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Electron Distribution in BaTi0.98Zr0.02O3 Piezoceramic Using X-ray Structure Factors 1 J. Mangaiyarkkarasi1, S.Sasikumar2, R. Saravanan2 1 – PG and Research Department of Physics, NMSSVN College, Nagamalai, Madurai, India 2 – Research centre and PG Department of Physics, The Madura College, Madurai, India DOI 10.2412/mmse.81.72.958 provided by Seo4U.link

Keywords: barium titanate, X-ray diffraction, structure factor, maximum entropy method, charge density.

ABSTRACT. Single phased BaTi0.98Zr0.02O3 piezo ceramic has been synthesized by conventional high temperature solid state reaction technique at 1450 oC for 10 hrs. and characterized. Precise electronic structure and charge density distributions of BaTi0.98Zr0.02O3 have been completely analyzed through powder X-ray diffraction data (PXRD). Powder profile refinement clearly evidenced that, the prepared ceramic has been crystallized in cubic perovskite structure with space group symmetry Pm 3 m. Average grain size is calculated by Scherer formulation. The bonding nature and electron distribution around Ba and O and Ti and O are examined by adapting maximum entropy method (MEM). The predominant ionic nature of Ba-O bond and the partial covalent nature of Ti-O bond are revealed by MEM qualitatively as well as quantitatively. The optical band gap energy has been estimated as 3.11 eV from UV-vis absorption spectroscopy. Surface morphology and microstructure are also analyzed by scanning electron microscopy (SEM). Particles with irregular shapes are observed from SEM image. Atomic percentages of chemical compositions of the synthesized ceramic are further confirmed by energy dispersive X-ray spectroscopy (EDS).

Introduction. Recently, the interest towards lead-free piezoelectric ceramic materials has been increasing for electromechanical transducer devices [1]. Among them, barium zirconium titanate (BZT) ceramic has attracted great attention for its potential applications in the fabrication of microwave devices and piezoelectric transducers due to its high dielectric constant, low dielectric loss and large tunability [2]. BZT has been particularly used for multilayer ceramic capacitors (MLCCs). The addition of Zr at the lattice sites of Ti is known to be effectively changes the Curie temperature (TC) and also presents many interesting features in the dielectric and ferroelectric properties of BaTiO3 [3]. Moreover, Zr4+ ion is comparatively more stable than Ti4+ ion, hence the Ti doping at the lattice sites of Zr would depress the conduction, thereby reducing the leakage current in the BaTiO3 structure [4]. BZT ceramic materials exhibit promising infrared and optical properties which are highly essential for designing pyroelectric and electro-optical devices [5]. Microwave dielectric properties of these Zr doped BaTiO3 materials also find applications in storage capacitors for the next DRAM generation, FeRAMs and non-volatile random access memories [6]. Even though many researchers have reported the structural and dielectric related investigations, the precise electronic structure, chemical bonding and charge density distribution studies are lacking in literature.

The detailed knowledge of the internal electronic structure of a material is extremely beneficial to understand the properties more clearly [7]. In this aspect, the present study focuses more on the bonding interactions between the constituent atoms of the BZT ceramic system. The accurate electronic structure of any crystalline material can be successfully elucidated by constructing the electron density from the X-ray structure factors using less biased mathematical tool such as maximum entropy method (MEM) [8].

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© 2017 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Experimental. BaTi0.98Zr0.02O3 ceramic has been synthesized by conventional high temperature solid state reaction technique. The stoichiometric mixtures of high purity starting materials BaCO3 (Alfa aesar, 99.997%), ZrO2 (Alfa aesar, 99.99%) and TiO2 (Alfa aesar, 99.99%) were thoroughly mixed using agate mortar and pestle. The mixed powder compound was calcined at 1200 oC for 2 hrs. in alumina crucibles using tubular furnace. Then the calcined powder was ground well using ball mill at 200 rpm for 5 h and compressed into dense pellets. These pellets were sintered to a high temperature of 1450oC with a dwell time of 10 h at a heating rate of 5o C/min in air and then they were slowly cooled at a normal cooling rate. The resultant sintered sample was finally ground well as smooth powder for characterization studies.

The synthesized sample has been structurally characterized by powder X-ray diffraction (PXRD) data sets collected at Sophisticated Analytical Instrument Facility (SAIF), Cochin University, Cochin, India using X-ray diffractometer (Bruker AXS D8 advance) with monochromatic CuKα radiation (λ=1.54056Å ), in the 2θ range of 10º-120º with the step size of 0.02º. Optical band gap has been evaluated from the UV-vis data obtained in the range of 200 nm-2000 nm using UV-vis spectrometer (Cary 5000, Varian, Germany). SEM image was recorded using scanning electron microscope (Carl Zeiss Evo 18) to analyze the surface morphology and microstructure. EDS results were also obtained using Energy dispersive X-ray spectrometer (Quantax 200 with X-flash-Bruker) to confirm the elemental compositions at International Research Centre, Kalasalingam University, Krishnankoil.

Result and discussions.

Powder X-ray diffraction analysis and structure refinement. The raw powder XRD pattern for the synthesized ceramic BaTi0.98Zr0.02O3 is shown in the figure 1. The sharp, well defined Bragg peaks indicated that the synthesized ceramic possess long range of crystalline nature. The prepared ceramic presents the cubic perovskite structure with the space group of Pm 3 m (space group number: 221) in agreement with the corresponding Joint Committee on Powder Diffraction Standards (JCPDS) data base (PDF# 31-0174).

Fig. 1. Raw XRD profile of BaTi0.98Zr0.02O3.

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Fig. 2. Fitted XRD profile of BaTi0.98Zr0.02O3.

The structural studies has been carried out using the Rietveld refinement technique [9] using the software JANA 2006 [10]. The position coordinates (x, y, z) were taken as (0, 0, 0) for barium, (0.5, 0.5, 0.5) for titanium and (0.5, 0.5, 0) for oxygen from standard Wyckoff position table [11].

Structural parameters along with the profile parameters, asymmetry, background and some other correction parameters related to the XRD pattern were also refined to minimize the error between experimentally observed and theoretically built profiles. Figure 2 represents fitted profile of BaTi0.98Zr0.02O3 using Rietveld [9] method. Refined profile confirms the better fitting between the experimental and calculated profiles.

Table 1. Refined parameters from Rietveld refinement of BaTi0.98Zr0.02O3.

Parameters

Values

a=b=c (Å)

4.0102(10)

α=β=γ (°)

90

Volume (Å3)

64.49(1)

Density (gm/cc)

6.02(1)

Profile reliability factor, RP (%)

6.94

Observed profile reliability factor, Robs (%)

3.19

Goodness of fit, GOF

1.27

Number of electrons in the unit cell, F(000)

102

The refinement provides satisfactory agreement factors and the structural parameters which are listed in table 1. The lattice parameter value is 4.0102 Å and cell volume is 64.49 Å. The average grain size of the prepared ceramic was calculated through Scherer formula [12]: t = 0.9λ / β cosθ, where t – is grain size; λ – is wavelength of X-ray; β – is the full width at half maximum (FWHM); MMSE Journal. Open Access www.mmse.xyz

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

θ – is the Bragg angle.

The average grain size is calculated as 26 nm.

Electron density and bonding investigations using MEM method. The precise electronic structure analysis can be done effectively by maximum entropy method (MEM) [10], which provides a more clear understanding and visualization of bonding features. The X-ray structure factors retrieved from Rietveld [9] method were utilized for MEM [10] refinement. Since the prepared system is crystallized in cubic structure, the unit cell was divided into 64×64×64 pixels. The prior charge density assigned to each pixel is F000/a3. The software package PRIMA [13] was employed for the numerical MEM computations and then the electron density maps are plotted using VESTA [14].

Fig. 3. (a) 3D unit cell with (100) plane shaded, (b) 2D electron density distributions of BaTi0.98Zr0.02O3 on the (100) plane, (c) enlarged view of bonding between Ba and O atom.

To understand the nature of bonds along Ba-O and Ti-O bond paths, 2-dimensional charge density contour maps are constructed for two different miller planes (100) & (200) with the contour range of 0 e/Å3 to 1 e/Å3, and the contour interval of 0.04 e/Å3. Figure 4(a) shows the 3D unit cell with shaded (100) plane. The positions of the constituent atoms Ba, Ti and O are distinctly visualized, in which Ba atoms are at the corners of the cube, Ti atom is at the body center, and the O atoms are at the face centers. Figure 3 (b) and 3 (c) demonstrate the 2D contour maps upon (100) plane and enlarged bonding portion between Ba and O atoms respectively. Figure 4 (a) shows the 3D unit cell with shaded (200) plane. There is no sharing of valence charges are seen between Ba and O atoms, which indicates the ionic nature of Ba-O band. Figure 4 (b) and 4 (c) demonstrate the 2D contour map upon (200) plane and enlarged bonding portion between Ti and O atoms respectively. The charge density contours between the Ti and O atoms are overlapping which authenticates the covalent nature of TiO bond.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Fig. 4. (a) 3D unit cell with (200) plane shaded, (b) 2D electron density distributions of BaTi0.98Zr0.02O3 on the (200) plane, (c) enlarged view of bonding between Ti and O atoms.

The accurate values of bond lengths and numerical values of electron densities for Ba-O and Ti-O bonds are calculated by drawing one dimensional line profiles. Figure 5 and 6 represent the one dimensional line profiles for Ba-O and Ti-O respectively. The bond length of Ba-O bond is 2.8357 Å and the bond length for Ti-O bond is 2.0051 Å. The electron density at bond critical point (BCP) between Ba and O is 0.2509 e/Å3. The minimum electron density value confirms the ionic nature between Ba and O ions. The electron density at bond critical point (BCP) between Ti and O is 0.6197 e/Å3, which confirms the covalent nature between Ti and O ions.

Fig. 5. 1D line profile for Ba-O bond.

Fig. 6. 1D line profile for Ti-O bond.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Band gap evaluation. Band gap energy of BaTi0.98Zr0.02O3 has been evaluated by the procedure proposed by Tauc et al., [15] using UV-Vis data. A graph was drawn by taking energy values (hν) in X-axis and (αhν)2 in the Y-axis shown in figure 7. The band gap energy has been evaluated by extrapolating the linear portion of the curve to X-axis. The band gap energy for the synthesized sample is 3.11 eV.

Fig. 7. UV-Visible plot of BaTi0.98Zr0.02O3.

SEM/EDS studies. Surface morphology and the microstructure of the prepared sample have been investigated by scanning electron microscopy (SEM). SEM micrograph corresponding to ×25000 magnifications is shown in figure 8.

Fig. 8 SEM image of BaTi0.98Zr0.02O3.

Fig.9 EDS spectrum of BaTi0.98Zr0.02O3.

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Particles with non-uniform and irregular shapes and sizes are clearly visualized from this figure. The EDS spectrum is given in figure 9 confirms the presence of Ba, Ti, Zr and O atoms and no other additional peaks are seen. The stoichiometry of the prepared sample is given in table 2.

Table 2. Elemental compositions from EDS Element

Atom(%)

Weight(%)

Ba

34.49

77.32

Zr

10.07

7.87

Ti

0.27

0.40

O

55.17

14.41

Summary. BaTi0.98Zr0.02O3 ceramic material has been synthesized through high temperature solid state reaction method and analysed. Powder profile refinement confirms that the sample has been crystallized in cubic perovskite structure with single phase. The precise electronic structure, bonding interactions and electron distribution around Ba and O and Ti and O have been investigated through maximum entropy method (MEM). The predominant ionic nature of Ba-O bond and the partial covalent nature of Ti-O bond are revealed by MEM calculations. The optical band gap energy has been evaluated as 3.11 eV from UV-vis absorption spectroscopy. Particles with irregular shapes and sizes are clearly visualized from SEM image. Atomic percentages of chemical compositions of the synthesized ceramic are further confirmed by EDS spectrum.

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